Aging methods for bones

Skeletochronology is a method to determine the absolute age of an animal by growth cycles in compact bone (Castanet et al., 1977). In many recent tetrapods, one growth cycle commonly represents one year of time (e.g. Castanet et al., 1993; Chinsamy-Turan, 2005; Francillon-Vieillot et al., 1990:508; Hutton, 1986; Klevezal, 1996; Klevezal & Kleinenberg, 1969; Peabody, 1961; Ricqles et al., 1991:38). Thus, this knowledge was used to estimate age in many extinct tetrapods (e.g. Botha & Chinsamy, 2000; Chinsamy, 1990; Erickson & Brochu, 1999; Erickson & Tumanova 2000; Horner & Padian, 2004; Sander et al., 2006; Varricchio, 1993). However, there are many problems with this method one have to consider before its application.

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First, one has to be sure, if preserved lines are true annuli or LAG’s. Resorption or reversal lines are different from LAG’s or resting lines, because they are the border between the surface of an old, formerly resorbed bone tissue and newly deposited bone tissue (Francillon-Vieillot et al., 1990), for example during relocation of metaphyseal bone tissue into the diaphysis or during a change of the direction of osseous drift. A special but widely distributed type of such lines is the cementing line enclosing the lamellae of secondary osteons. Annuli and LAG’s can be, thus, distinguished from resorption lines by their cyclicity (always following a growth zone), by their relatively consistent and regular appearance, and they do not separate bone tissues of different developmental origin from each other.

The next problem is to make an accurate count of the number of annuli/LAG’s to get an age estimation for the respective animal. There has to be considered that the ontogenetic expansion of the marrow cavity can resorb several of the innermost growth cycles and that secondary remodeling is able to completely obscure most of the zonal primary compact bone. A possible solution is the back-calculation of the lost/obscured number of annuli/LAG’s (e.g. Castanet et al., 1993:265), where the distances between successive annuli/LAG’s were measured and, by using several statistical methods, the pattern of intervals were extrapolated into the marrow cavity and/or into the remodeled area (e.g. Horner & Padian, 2004; Klein, 2004; Werning, 2005). If available, the ontogenetic series of the bone of an animal and the superposition of them are another possibility to get absolute age estimates. The abundant variation in the distances between successive annuli/LAG’s or the same variation between two cycles in different parts of the cross section are here less significant, because the annuli/LAG’s of earlier stages are definitely known by younger specimens (e.g. Chinsamy, 1993; Erickson & Tumanova, 2000; Horner et al, 2000).

It is furthermore important to know that there is a high variability of the LAG number observed in different individuals of a population (e.g. Klevezal, 1996), between different skeletal elements of one individual (e.g. Horner et al., 2000), and sometimes even in the cross section of a single bone (e.g. Ricqles, 1983). For example, single individuals of the dinosaurs Hypacrosaurus

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(Horner et al., 1999), Maiasaura (Horner et al., 2000), and Plateosaurus (Klein, 2004) show different numbers of preserved LAG’s in different skeletal elements, which obviously depends on the general anatomical condition and specific growth pattern of each of these elements (e.g. cortical thickness, growth rate, rate of remodeling etc.).

A last important point on this topic is the assumption, whether all annuli/LAG’s counted in a bone are indeed true annual layers. It is known that recent tetrapods also generate them in case of very uncomfortable environmental conditions, such as scarcity of food or illness, or during seasons of pairing or reproduction (Castanet et al., 1993). It is also possible to find double LAG’s, which are consistently close together and represent a single year, and e.g. some tropical mammals can even generate two cycles in one year (Klevezal, 1996). All these deviations from the simple annual model of growth cycles are not discernable in extinct species and must be treated as sources of error in the calculation of individual age.

Amprino (1947) suggested that similar bone tissues in different animals represent similar growth rates. This assumption was used afterwards to estimate growth rate in extinct animals independently of skeletochronology (e.g. Horner et al., 2000). It is now widely accepted that adult or maximum body size seems to be one of the major factors, which have influence onto growth rate and therefore indirectly on bone tissue types (Buffrenil et al., 2008; Castanet et al., 2000; Erickson et al., 2004; Padian et al., 2004; Turvey et al., 2005). There are also differences in growth rate between different elements of a single skeleton (e.g. Buffrenil et al., 2008; Horner et al., 2000; Klein, 2004; Starck & Chinsamy, 2002) and during ontogeny (e.g. Castanet et al., 1993; Chinsamy, 1995; Horner et al., 1999; Werning, 2005). However, quantitative tests were extraordinarily rare, so that the accuracy of ‘Amprino’s Rule’ was not exactly proofed. This was finally done by a few recent studies on some birds and reptiles, which show a clear correlation between the size and density of vascular canals within the periosteal bone matrix, but no correlation with the orientation of vascular canals (Buffrenil et al., 2008; Castanet et al., 2000; Margerie et al., 2002; Starck & Chinsamy, 2002). A correlation of growth rate with vascular orientation seems to exist only due to extreme environmental conditions,

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which force an animal to generate extraordinarily high growth rates (Margerie et al., 2004). Thus, ‘Amprino’s Rule’ can help to estimate the growth rate of an extinct species, but, as in the method of skeletochronology, the results are strongly dependent on body size, ontogenetic stage, and skeletal element and should always be considered in comparison between different individuals, populations, and species.